It is conventional to use propellers that are arranged on a lift surface of an aircraft in order to propel it. By expelling the air passing through it, each propeller creates a propulsive or tractive force suitable for causing the aircraft to move forwards in translation. Thus, a propeller generally comprises a hub secured to a plurality of blades, the hub being covered by a conical fairing commonly called a “spinner”.
In the early days of aviation, propellers were provided with a plurality of fixed-pitch blades, the blades and the hub forming a single block. Such propellers are therefore referred to as “fixed-pitch propellers”. The pitch of the blades was consequently set permanently at a value that was determined at the time the propeller was fabricated. Depending on the intended mission, a pilot could choose to fit a propeller of small pitch for enhanced climbing, or a propeller of large pitch for enhanced cruising flight. In addition to it not being possible to have a propeller that is optimized for all missions, it will readily be understood that such propellers present difficulties in that the entire propeller needs to be changed in order to go from one configuration to another.
A significant improvement to that type of propeller is known. It comprises a second type of propeller known as a propeller of pitch that is variable on the ground. The pitch of the blades of a propeller can thus be adjusted on the ground. By unlocking a blade clamping collar, the blades can be pivoted into the desired position in order to change the pitch. Compared with the first type of propeller, the second type avoids the need to remove the propeller. Such adjustment, however, can clearly not be performed in flight.
A third type of propeller, known as a “variable pitch propeller” has also been implemented. The aircraft then has a system for controlling pitch variation that enables the pitch of the propeller blades to be changed in flight. Conventionally, such a pitch variation control system has a hydraulic pump that is activated by the pilot via a control, a hydraulic chamber provided under the conical spinner of the propeller, and a piston associated with the blades via a rod.
Depending on the order issued by the pilot, the pump injects fluid into the hydraulic chamber via flexible pipe. The resulting change of pressure in the hydraulic chamber causes the piston to move. The blades are then caused to turn about their pitch variation axes by the piston. That third type of propeller thus enables blade pitch to be varied while in flight so as to go from a small pitch on takeoff to a large pitch while cruising.
It should also be observed that in the event of the engine installation that drives a propeller in rotation breaking down, the blades can be oriented in such a manner as to minimize their relative wind resistance and thus minimize drag. This is referred to as “feathering” the propeller. Nevertheless, this third type of propeller is not totally satisfactory. If, during flight, the pilot pulls up the nose, the speed of rotation of the propeller drops and the aircraft loses speed.
Consequently, a fourth type of propeller has been implemented to maintain optimized propulsion, or optimized traction, as a function of the orientation of the propeller, this fourth type being referred to as a “constant speed” propeller. As with the third type, a hydraulic device is provided to vary the pitch of the propeller blades in flight. Furthermore, the pilot now controls a throttle to adjust the power delivered by the aircraft engine installation. Regulator means are implemented and control both the power from the engine installation and the pitch of the blades so as to maintain the speed of rotation of the propeller constant. Optionally, the aircraft includes a propeller control used by the pilot to set said speed of rotation of the propeller.
The pitch variation control system used by the third and fourth types of propeller is effective. Nevertheless, it can be difficult or even impossible to implement it. If the hydraulic fluid feed needs to pass via the power transmission shaft driving the propeller in rotation, then it is not possible to use a flexible pipe, which is needed to perform a rotary movement.
Similarly, for safety reasons, it can be necessary to duplicate pitch variation systems, which can be difficult. Consequently, the assembly comprising a pump and a flexible pipe has been replaced by a hydraulic slide valve and a longitudinally-pierced bar. The bar is constrained to rotate with the propeller, and it is fed via the hydraulic valve. The valve then fills the longitudinal borehole in the bar with fluid, e.g. oil, so as to cause the piston in the propeller to move and vary the pitch of the propeller's blades. Nevertheless, modern techniques enable boreholes to be made in bars of small dimensions only, with the piercing of bars of large dimensions leading to completely unacceptable departures from alignment. Conventionally, the person skilled in the art thus machines bars of small dimensions and assembles such bars together by welding them in pairs, the junction zones being covered by sleeves. This solution is burdensome and difficult to implement. Furthermore, it is not possible to pierce a large number of channels in a bar of small dimensions, thus limiting the quantity of fluid that can be conveyed by the transmission shaft and preventing the control system from being made redundant.